Although the U4+ ion spectrum in crystalline matrix has been observed since 1960, the first results on the free-ion (U V) have been obtained in 1980, from the lines still unidentified after the preliminary analysis of U VI (80.5). The study of the sliding spark spectrum in different conditions allowed to discriminate U V lines from those of U IV and U VI. The most intense of these lines have led to the determination of seventy five energy levels, most of them belonging to the two-electron spectrum. Configurations 5f2, 5f6d, 5f7s and the group 5f7p + 6d2 are well interpreted by the Slater-Condon method. New even levels which are evidently due to the excitation of the complete 6p6 shell are found above 145870 cm-1, mixed with those of 5f7p and 6d2. These levels identified as 6p55f3, combine with nine high lying odd levels which could belong to the configuration 6p55f26d.
The twelve known levels of 5f2 have led Goldschmidt (83.3) to a first determination of the effective magnetic interaction parameters issued from the second order of the perturbation theory. The last missing level (1So) of this configuration has been located by van Deurzen, Rajnak and Conway with the help of a new theoretical study and of the observation of the ultraviolet spectrum between 200 and 700 nm (84.5).
At the present time, 264 lines of U V have been classified (80.5; 84.5).